The invention is an improvement in the catalytic cracking process.
Catalytic cracking of hydrocarbons with zeolite-containing catalyst is a well-known process.
In fluidized catalytic cracking, the hydrocarbons contact fluidized catalyst and are cracked to lighter products.
The catalyst is deactivated by coke deposition, necessitating regeneration of coked catalyst in a regenerator.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by still smaller channels. These materials are known as molecular sieves.
Many methods have been developed for preparing a variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other symbol, e.g., Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. No. 2,882,244), Zeolite Y (U.S. No. 3,130,007), and ZSM-5 (U.S. No. 3,702,886), merely to name a few.
ZSM-5 is a particularly interesting zeolite. Much work has been reported in the patent literature on adding ZSM-5 to the conventional cracking catalyst in a catalytic cracking unit.
In U.S. Pat. No. 3,758,403, from 2-1/2 to 10 wt. % ZSM-5 catalyst was added to a conventional cracking catalyst containing 10 percent REY, the remainder being Georgia clay. Examples were given showing use of 1.5, 2.5, 5 and 10 wt. % ZSM-5 added to the conventional cracking catalyst.
The shape selective zeolites, e.g., ZSM-5, increase production of LPG (C.sub.3 /C.sub.4 olefins) in cracking units and cause some loss of gasoline yield, and an increase in octane number.
ZSM-5 catalyst, especially virgin catalyst, has exceedingly high activity. Researchers have attempted to take advantage of the super activity of fresh ZSM-5 catalysts by adding only small amounts of it to FCC catalyst. Such work is shown in U.S. Pat. No. 4,309,280. This patent taught adding very small amounts of powdered, neat ZSM-5 catalyst, characterized by a particle size less than 5 microns.
This patent taught that adding as little as 0.25 weight percent ZSM-5 powder to the circulating catalyst inventory in an FCC unit would increase wet gas or LPG production by 50 percent.
Because of the high initial activity of the ZSM-5 catalyst, and increased wet gas production due to the addition of large amounts of ZSM-5, refiners are reluctant to start up a unit with, or add quickly to a unit, a relatively large amount of virgin ZSM-5 catalyst.
If only very small amounts of ZSM-5 catalysts are added, there will be no problem with excessive wet gas production, but the effect of adding ZSM-5 may not be apparent for days or weeks, by which time normal variations in feed, or unit operation, may obscure the octane enhancing affect of ZSM-5 addition.
A good way to add a modest amount of ZSM-5 to an FCC unit is disclosed in EP 0168979, published European Application No. 85304367.7 (F-2955 EPO). The method involves adding ZSM-5 catalyst to the equilibrium catalyst in an FCC unit in a programmed manner so that an immediate boost in gasoline product octane number, typically 178 -2 octane number, is achieved.
Although the method works well in practice, it requires careful monitoring of ZSM-5 addition over a period of many days, and results in only a modest amount of ZSM-5 catalyst being added to the equilibrium catalyst. The upper limit on ZSM-5 addition taught in the published European Application is typically enough ZSM-5 to achieve an immediate 2.0 octane number gainm with the possibility being left open to add enough ZSM-5 to go to a 3.0 octane number gain. The limiting factor is the ability of the wet gas compressor of the catalytic cracking unit to deal with the enhanced wet gas production associated with adding large amounts of ZSM-5.
A good way of adding ZSM-5 to a moving bed catalytic cracking unit is disclosed in published European Application No. EP 0167325A3. The make-up catalyst may contain 2 or 3 times the amount of ZSM-5 sought for the equilibrium catalyst.
Accordingly, refiners have not been able to add as much ZSM-5, or other shape selective zeolite additives which behave in a similar manner, as desired because of the constraints placed on such addition by the unit's wet gas compressor.
A way has now been discovered to overcome this restraint, and add relatively large amounts of shape selective zeolites, such as ZSM-5, to a catalytic cracking unit without exceeding the compressor limits.